From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Smalley Subject: Re: [RFC] The reflink(2) system call v4. Date: Tue, 12 May 2009 13:32:47 -0400 Message-ID: <1242149567.31807.90.camel@localhost.localdomain> References: <1241331303-23753-1-git-send-email-joel.becker@oracle.com> <20090507221535.GA31624@mail.oracle.com> <4A039FF8.7090807@hp.com> <20090508031018.GB8611@mail.oracle.com> <20090511204011.GB30293@mail.oracle.com> <20090511223414.GA28209@mail.oracle.com> <1242130714.31807.25.camel@localhost.localdomain> <20090512172200.GC6896@mail.oracle.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: James Morris , jim owens , ocfs2-devel@oss.oracle.com, viro@zeniv.linux.org.uk, mtk.manpages@gmail.com, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Joel Becker Return-path: Received: from msux-gh1-uea02.nsa.gov ([63.239.67.2]:61549 "EHLO msux-gh1-uea02.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751151AbZELRj4 (ORCPT ); Tue, 12 May 2009 13:39:56 -0400 In-Reply-To: <20090512172200.GC6896@mail.oracle.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, 2009-05-12 at 10:22 -0700, Joel Becker wrote: > On Tue, May 12, 2009 at 08:18:34AM -0400, Stephen Smalley wrote: > > On Tue, 2009-05-12 at 11:12 +1000, James Morris wrote: > > > On Mon, 11 May 2009, Joel Becker wrote: > > > > > > > > e.g. SELinux will need to perform some checks on the operation, then > > > > > calculate a new security context for the new file. > > > > > > > > Do I need to pass in preserve_security as well so SELinux knows > > > > what the ownership check determined? > > > > > > Not for SELinux -- its security attributes are orthogonal to DAC, and it > > > will perform its own checks on them. > > > > Is preserve_security supposed to also control the preservation of the > > SELinux security attribute (security.selinux extended attribute)? I'd > > expect that either we preserve all the security-relevant attributes or > > none of them. And if that is the case, then SELinux has to know about > > preserve_security in order to know what the security context of the new > > inode will be. > > Thank you Stephen, you read my mind. In the ocfs2 case, we're > expecting to just reflink the extended attribute structures verbatim in > the preserve_security case. And in the preserve_security==0 case, you'll be calling security_inode_init_security() in order to get the attribute name/value pair to assign to the new inode just as in the normal file creation case? > So we would be ignoring whatever was set on > the new_dentry by security_inode_reflink(). This gets us the best CoW > sharing of the xattr extents, but I want to make sure that's "safe" in > the preserve_security case. security_inode_reflink() can't handle the initialization regardless, as the inode doesn't yet exist at that point. > > Also, if you are going to automatically degrade reflink(2) behavior > > based on the owner_or_cap test, then you ought to allow the same to be > > true if the security module vetoes the attempt to preserve attributes. > > Either DAC or MAC logic may say that security attributes cannot be > > preserved. Your current logic will only allow graceful degradation in > > the DAC case, but the MAC case will remain a hard failure. > > I did not think of this, and its a very good point as well. I'm > not sure how to have the return value of security_inode_reflink() > distinguish between "disallow the reflink" and "disallow > preserve_security". But since !preserve_security requires read access > only, perhaps we move security_inode_reflink up higher and say: > > error = security_inode_reflink(old_dentry, dir); > if (error) > preserve_security = 0; > > Here security_inode_reflink() does not need new_dentry, because it isn't > setting a security context. If it's ok with the reflink, we'll be > copying the extended attribute. If it's not OK, it falls through to the > inode_permission(inode, MAY_READ) check, which will check for plain old > read access. > What do we think? I'd rather have two hooks, one to allow the security module to override preserve_security and one to allow the security module to deny the operation altogether. The former hook only needs to be called if preserve_security is not already cleared by the DAC logic. The latter hook needs to know the final verdict on preserve_security in order to determine the right set of checks to apply, which isn't necessarily limited to only checking read access. But we don't need the new_dentry regardless. -- Stephen Smalley National Security Agency